The present invention relates to a method for the manufacture of chromium containing, chill cast, cast iron camshafts.
Camshafts for use in, for example, internal combustion engines have been made in cast iron. There are two production methods which have been used most extensively, these are; either to cast the camshaft in a hardenable iron followed by, for example, induction hardening of the cam lobes, or to incorporate cold metal chills in the casting mould to produce a white iron chill cast structure in at least the cam lobes during the casting process. It is the latter production process with which the present invention is principally concerned.
Camshafts generally comprise an elongate shaft on which the valve operating cams are disposed in varying orientations together with camshaft bearing journals and also sometimes other features, such as ancillary equipment drive gears or various projections, for example, which require post-casting machining. Indeed, the shaft itself often requires a bore to be produced along the shaft centre, the bore usually being produced by the technique known as "gun-drilling". The white iron structure of cast iron is ideally confined to the cam lobe regions where it is desirable for its wear resistant properties which stem from the high hardness of this type of structure. White iron comprises iron carbides in a pearlite matrix; the iron carbides rendering the metal so hard that the cam lobes are normally finished by grinding. Where metal cutting operations need to be performed on portions of the cast camshaft it is desirable that such portions solidify as grey iron which has a structure comprising graphite flakes in a pearlite matrix and which is readily machinable by normal metal cutting techniques.
Which form of cast iron is produced on solidification will depend, principally, upon several factors which include the chemical composition of the iron being cast, the cooling rate of the metal during solidification and the degree of nucleation applied to the molten metal.
Co-pending British Patent application number 9106752.0 of common ownership herewith describes the manufacture of chill-cast camshafts from substantially unalloyed cast-iron. In some applications, it is necessary that the camshaft be made of an inherently stronger material than basic unalloyed cast-iron. For this reason chromium is frequently used as an alloying addition to cast-iron. Chromium increases the mechanical properties such as, for example, fatigue resistance, tensile strength, shear strength, torsional strength and hardness of grey iron. It is the grey iron constituent which gives the camshaft its strength and rigidity. Some engines, particularly diesel engines have auxiliary services driven from the camshaft. Such services might include a fuel injection pump and a hydraulic pump in the case of agricultural vehicles for example. Where drives for these services are taken from the camshaft, it is necessary that the material strength is significally greater than with unalloyed cast iron to limit twisting of the shaft in operation. The effect of chromium in increasing the hardness of grey iron is also important for the shaft bearing journals.
Vanadium has a similar effect to chromium, but it is very much more costly as a raw material and, therefore, its use tends to be sparing.
With conventional chill cast camshafts, white iron is produced at the cam lobe surface by the use of metal chills placed in the casting mould, which is generally composed of sand. The metal chills produce a sufficiently high cooling rate such as to ensure solidification of the molten cast iron as white iron adjacent the chills. A problem arises in some designs of camshafts where a particular feature, which requires subsequent machining, has a relatively low metal volume compared to the area of the adjacent sand mould material. In this instance the cooling rate produced by the sand mould itself may be sufficiently high to produce white iron in such features, thus causing machinability problems.
The formation of grey iron on solidification is a nucleation and growth reaction, the carbon atoms precipitate onto a suitable nucleating site, which may be an oxide or sulphide impurity particle or which may be a deliberately added nucleant material such as ferrosilicon or calcium silicide, for example, and grow as graphite flakes, usually in the form of "rosettes". The diffusion of carbon atoms through the solidifying metal to form graphite flakes takes time and, if there are relatively few nucleation sites, they have to travel further which increases the necessary time required for diffusion.
The effect of the requirement of time for diffusion is that, where there is a superimposed high cooling rate due to a chill insert or localised area of sand mould, insufficient diffusion time is available before the metal adjacent the chill becomes undercooled below the iron-cementite (iron carbide) formation eutectic temperature on the iron-carbon phase diagram and the iron solidifies in the metastable white iron form.
In the regions of the solidifying camshaft remote from the chills, the rate of cooling is far lower than that adjacent the chills, therefore, more time is available for the diffusion of carbon in the still molten iron and, by adjustment of the level of nucleation of the molten charge prior to pouring, these regions may be induced to solidify in the grey iron form. However, control of the level of nucleation is critical and too high a level may result in not meeting a specification for the minimum depth of white iron to be achieved and too low a level may result in a high proportion of white iron appearing in the grey iron regions, this again leading to machinability problems. A further disadvantage of this is that inconsistent mechanical properties will result in the grey iron parts of the camshaft; the grey iron parts may, for example, be too brittle.
A yet further disadvantage is that where a high level of nucleation is employed in order to overcome the machinability difficulties, due to kinetic factors associated with the eutectic solidification reactions promoted by the nonequilibrium thermal effect arising in the mould in practice, some grey iron cells may arise within the white iron structure causing a lowering of the hardness and, therefore, impairing the wear resistance of the material, which is undesirable. Adjustment of the nucleation to a lower level to prevent undesirable grey iron cells within the white iron regions results in a increase in white iron depth as well as promoting the formation of some white iron carbides within the desired grey iron region as described above. The net effect of this may be to make the production of a bore by gun-drilling infeasible, especially in camshaft designs requiring a 360 degree, full peripheral white iron zone.
The above problems are exacerbated by the presence of chromium (or vanadium) in the iron. The metallurgical effect of chromium is to simultaneously lower the iron-graphite eutectic temperature whilst at the same time markedly raising the iron-iron carbide eutectic temperature, the consequence of which is to make it easier to form the white iron phase by requiring much lower cooling rates when compared to chromium free alloys for any given cooling conditions since much less undercooling is necessary for this to occur. The presence of chromium increases the problem when thin sections are present in the casting in that the chilling effect of the mould sand can be sufficient to impart the degree of undercooling necessary to produce white iron, leading as stated above to machinability problems.
Some other elements, of which the most important is silicon, have a similar effect to carbon on the solidification of cast iron; 1 weight % of silicon has the same effect as 0.25 weight % of carbon. It is usual, therefore, to quote cast irons as having a "carbon equivalent" (CE) which is arrived at by adding together the total percentage of carbon and 0.25 .times. the total percentage of silicon. There are other elements, such as phosphorus for example, which have a carbon equivalent effect but are less important.
A yet further consequential problem of the chromium alloying addition is the tendency for some carbon to be taken up in the form of intercellular carbides, which in themselves may not be detrimental to machinability due to their morphology, but reduce the amount of carbon available to form graphite. In one respect this is beneficial in that it realises the hardness of the material but in a second respect is disadvantageous in that the solidifying metal becomes more prone to shrinkage defects. To counteract this shrinkage effect it has been common to raise the CE to compensate for the intercellular carbides and, in some cases, to increase the level of nucleation to prevent excessive white iron formation. Due to the criticality of the nucleation level, the resulting effect is often to produce free graphite in the white iron chill zones thus reducing the wear resistance and hardness of the white iron zone.
In the production of chilled iron camshafts heretofore, it has been common to employ a CE in the range from above 4.0 up to 4.3, this CE comprising about from 3.5 to about 3.9 wt % of carbon, the remainder silicon. This level of CE is close to the cast iron eutectic composition of 4.3. It has been customary, for economic reasons, to use a "self-feeding" iron owing to there being only one combined feeder and riser in the camshaft casting mould. All solidification shrinkage is fed from the liquid shaft core and, because of the need to feed from one end to the other of the whole shaft the shrinkage negating effect of the higher carbon level has been virtually mandatory.